Electronic Sensing and Current Drive Self-Healing “Skin”

What you’ll learn:

Why true human-like self-healing materials are hard to create.
How a multilayered material based on liquid-metal elastomer was used to find and fix faults.
How the final step of “cleaning up” was accomplished using electromigration.

The idea of self-healing components and systems isn’t new, as materials scientists and engineers have been attempting to emulate this capability of living creatures — including humans — for a long time. However, nearly all of these efforts aren’t self-healing in the true biological sense of the term. Instead, they usually aim to identify a failure and then implement a workaround or bypass scheme, such as redirecting a program to access an unused memory block or switching in a spare functional block rather than truly repair the defect.

Perhaps the most audacious and astounding example of this “redirect” was demonstrated by the Jet Propulsion Laboratory (JPL) team that still monitors Voyager 1 and Voyager 2, both launched in 1977 and now beyond our solar system at about 22 billion kilometers (15 billion miles) from Earth. In 2024, Voyager 1 was unable to send readable science and engineering data back to Earth, although the basic communication link still worked.

Despite the 22-hour, one-way propagation delay and leisurely 100-bits/s data rate, the team was able to diagnose the problem to a partial failure in one memory bank. They then redirected the on-board code to use several smaller, unused memory segments in place of the defective one.¹

Taking “Self-Healing” to the Next Level

While there have been some laboratory-level successes at true self-healing and there are even standard, commercially available metallized capacitors with this attribute, efforts continue to devise more advanced self-healing for damaged materials. Now, a team at the University of Nebraska-Lincoln has devised a layered material for a soft robotics technology that can identify damage from a puncture or extreme pressure, pinpoint its location, and autonomously initiate self-repair (Fig. 1).

Self-healing artificial muscle — 1. (A) Schematic illustration of the intelligent self-healing artificial muscle. (B) Exploded schematic illustration of the layered architecture and integration of advanced materials to enable the artificial muscle to sense damage and facilitate self-repair mechanisms without manual intervention or external healing mechanisms.

The team’s “muscle” actuator has three layers. The bottom one — the damage-detection layer — is a soft electronic skin composed of liquid-metal (LM) microdroplets embedded in a silicone elastomer. That skin is adhered to the middle layer, the self-healing component, which is a stiff thermoplastic elastomer. On top is the actuation layer that initiates the muscle’s motion when pressurized with water.

To begin the process, the team induces five monitoring currents across the bottom “skin” of the muscle, which is connected to a microcontroller and sensing circuit. Puncture or pressure damage to that layer triggers formation of an electrical network between the traces. The system recognizes this electrical footprint as evidence of damage and subsequently increases the current running through the newly formed electrical network.

As a result, that network can function as a local Joule heater, converting the energy of the electric current into heat around the areas of damage. After a few minutes, this heat melts and reprocesses the middle thermoplastic layer, which seals the damage and effectively self-heals the wound.

The last step is resetting the system back to its original state by erasing the bottom layer’s electrical footprint of damage. To do this, the team exploited the effects of electromigration, a process in which an electrical current causes metal atoms to migrate. (Note that this phenomenon is usually viewed as detrimental in metallic circuits because moving atoms deform and cause gaps or short circuits in a circuit’s materials, leading to device failure and breakage.)

Building a Soft Robotic Actuator

A soft robotic actuator was designed consisting of a silicone elastomer layer with a series of bladders bonded to a stiffer, inextensible thermoplastic elastomer (TPE) layer. The soft electronic skin involved embedding the LM droplets within a highly deformable elastomer matrix. This LM elastomer composite was then adhered to the TPE layer.

When the top silicone layer was inflated with water, the difference in strain between the extensible top layer and inextensible bottom layers causes the actuator to bend. The soft electronic skin contained four parallel traces that were used to detect the location of damage by monitoring the impedance between each trace (Fig. 2).

2. Intelligent self-healing artificial muscle: (A) Bottom view of the actuator and configuration of the damage detection layer with four traces to detect damage events. (B) The actuator was pressurized with dyed water and (C) punctured with a precision knife. D) This damage event caused damage to both the damage detection layer (between traces 2, 3, and 4) and self-healing TPE layer. Fluid is shown leaking from the actuator. (E) The actuator was depressurized and a current was applied between traces 2 and 3 to melt the self-healing TPE layer and seal the puncture. (F) The current was then increased, causing electromigration and thermal failure to reconfigure the electrical network. (G) The actuator was pressurized again and functioned as expected. (H) The damage detection layer was fully reset by applying a current ramp between trace 3 and 4 until electromigration and thermal failure occurred, reconfiguring the electrical network.

If continuity was detected, the newly formed electrical network has a high resistance relative to the four monitoring traces, allowing the network to function as a local Joule heater. The Joule heating element can be used to locally heat the punctured area for several minutes to melt the TPE layer and self-heal the puncture damage. This enables the actuator to continue to operate as before.

The Electromigration Effect

Researchers realized that they could adapt electromigration to solve a problem that has long plagued their efforts to create an autonomous, self-healing system: The apparent permanence of the damage-induced electrical networks in the bottom layer. Without the ability to reset the baseline monitoring traces, the system can’t complete more than one cycle of damage and repair.

This electromigration — with its ability to physically separate metal ions and trigger open-circuit failure — might be the key to erasing the newly formed traces. The strategy worked: By further ramping up the current, the team was able to induce electromigration and thermal failure mechanisms that reset the damage detection network (Fig. 3).

Electromigration was used to reset the damage-detection function — 3. This simplified schematic diagram shows how the principle of electromigration was used to reset the damage-detection function.

Eric Markvicka, assistant professor of biomedical engineering and project leader, noted that “…electromigration is generally seen as a huge negative. It’s one of the bottlenecks that has prevented the miniaturization of electronics. We use it in a unique and really positive way here. Instead of trying to prevent it from happening, we are, for the first time, harnessing it to erase traces that we used to think were permanent.”

The work was supported by the National Science Foundation, the NASA Nebraska Established Program to Stimulate Competitive Research, and the Nebraska Tobacco Settlement Biomedical Research Development Fund. The work is discussed in in detail in their paper “Intelligent Self-Healing Artificial Muscle: Mechanisms for Damage Detection and Autonomous Repair of Puncture Damage in Soft Robotics” which was presented at the 2025 IEEE International Conference on Robotics and Automation.